Electric furnace with carbonaceous atmosphere



May 1, 1956 s. R. WHITE, .JR

ELECTRIC FURNAcE WITH cARBoNAcEoUs ATMOSPHERE Filed OCC. 28. 1954Invenfor: Samuel R.Whi{e Jr. lay/.WMA Mn-@ His AHorney United StatesPatent 2,744,152 ELECTRIC FURNACE lll/HH CARBNACEUS ATMQSPHERE Samuel R.White, Jr., Baliston Spa, N. Y., assigner to General Electric (lomp-any,a corporation of New Yorlr Application October 2S, l'f'd, Serial No.fe'flfsf 6 Claims. (Cl. 13-2tl) My invention relates to electricalfurnaces and in particular to furnaces of the controlled atmosphere typein Which the various heating and heat treatment processes are carriedout with the charge or charges therein enveloped in an atmosphere ofcarbonaceous gases.

As is well lqnown in the art, carbonaceous atmospheres of compositionsmost frequently employed in applications of the aforementioned typegenerally contain a low percentage of carbon dioxide and a relativelyhigh percentage of carbon monoxide. lt has been observed that in anycarbonaceous atmosphere containing any appreciable amounts of carbonmonoxide without oxygen being present, free carbon tends to drop out andbuild up on the heated metallic furnace parts within the furnaceenclosure. This free carbon drop out reaction tends to be quite severeover the temperature range of operation of about 800 F. to l300 F., themost critical range being in the vicinity of 900 F. to 1100 F., with thecarbon deposits being particularly objectionable on the current carryingelements and electrical terminal members within the furnace walls Where,in a very short time, these deposits become heavy enough to form shortcircuits between adjacent terminal members or closely positionedconductor members and between such members and the furnace casing.

Under normal operating conditions in temperature ranges normallyemployed in such furnaces, the electrical conductors and terminalsextending through the furnace walls, pass through a temperature zonewhere the free carbon formation reaction is very severe, and it istherefore with respect to elements in this category in particular thatthe problem of free carbon deposits has been most pressing.

Such deposits generally do not tend to form on the furnace heating unitsand the highly heated parts withi the high tern erature zones of thefurnace, since these elements are normally out of the temperature rangewhere such deposits form at an objectionable rate. However, inapplications where it may be desired to operate the heating elements ina furnace with a carbonaceous atmosphere within the temperature range ofabout 800 l?. to approximately l300 F., carbon deposits would build upon these elements at an obiectionably rapid rate thereby forminginsulating layers on the heaters which adversely affect the heattransfer characteristics thereof.

The above reaction may be represented in chemical symbols as follows:

The problems created by carbon drop out in furnaces having carbonaccousatmospheres have existed fo` some time and various attempts have beenmade to alleviate the effects of such deposits, such as for instance byenclosing the electrical terminals, conductor elements, and heaters ofsuch furnaces in protective housings, which housings were insulated fromthe conducting elements so that the carbon deposits could form on thehousings without danger of short circuits. With such arrangements,however, the carbo-rx drop out was still not prevented and consequentlyprovision had to be made for allowing removal of such deposits from the`protective housings at regular intervals in order to prevent excessivebuildups from interfering with the heat transfer characteristics of thefurnace and from mechanically hampering and imped- Mice ing operationthereof. In addition, the provision of such protective housings becomesrather diicult in certain configurations and adds to the manufacturingcost of the furnace.

ln view of the foregoing, it is, accordingly one object of my inventionto provide means for greatly reducing the rate of carbon drop out onfurnace parts in furnaces having carbonaceous atmospheres.

lt is another object of my invention to provide an improved electricalconductor for use in furnaces having carbonaceous atmospheres, whichimproved conductor is not subject to the aforementioned disadvantage ofexcessive carbon formation on the outer surfaces thereof.

l provide, in accordance with one aspect of my invention, means forgreatly reducing carbon deposits on metallic surfaces within furnaceshaving carbonaceous atmospheres therein. l have discovered that certainmetals, in particular those commonly employed and accepted as being themost suitable for use as electrical terminals and conductor elementsfrom the standpoint of electrical resistance characteristics and costand also those commonly employed in outer sheaths of sheathed heaters,tend to act as catalysts to the above reaction and that certainmaterials such as chromium, are non-catalytic to the carbon drop outreaction. In other words, materials most commonly employed and mostsuitable for use in heaters and electrical terminals and conductors tendto greatly accelerate the reaction causing carbon drop out whereascertain other metals, such as chromium do not have this effect so thatcarbon deposits on such metals form at such a low rate as to eliminatethe problem.

lily invention therefore contemplates, in accordance with one aspectthereof, the utilization of an electrical conductor or terminal memberhaving an outer surface which is non-catalytic to the above-mentionedcarbon drop out reaction. embodiment of my invention illustrated herein,I prefer to employ, from the standpoint of providing the optimum inelectrical characteristics as Well as satisfying cost considerations, acomposite conductor element having a central conducting core formed of amaterial chosen from the standpoint of its electrical characteristics,with the outer non-catalytic surface being formed by a coating orplating of a non-catalytic material, such as chromium. It will be seenthat my invention is particularly applicable to the current carryingconductors, terminals and heaters passing through the furnace walls andalso to those elements within the furnace enclosure which operate Withinthe above mentioned temperature ranges, since the formation of carbondeposits on Xtures of this type has long been a serious and particularlypressing problem.

@ther objects and advantages of my invention will be apparent from thefollowing detailed description taken in connection with the accompanyingdrawing, and its scope will be pointed out in the appended claims.

Referring to the drawing, Fig. l is a front cross sectional vicw of afurnace utilizing a carbonaceous atmosphere and illustrating oneembodiment of my invention; Fig. 2 is a side cross sectional view of thefurnace of Fig. l; while Figs. 3 and 4 are side and respectively endenlarged views of one of the terminals of the furnace of Figs. l and 2illustrating the non-catalytic plating thereon.

Referring to Figs. l and 2, there is shown a furnace of the typecommonly referred to as a box furnace which comprises an outer housing 1and an access door 2 as shown in Fig. 2. The furnace housing 1 and door2 are provided with a iirst lining of heat insulating material 3 and asecond inner lining of a more refractory heat insulating material 4.

The charge is supported in the furnace on a hearth plate 5 and is heatedby means of alloy ribbon resistors 6 runln accordance with theparticular" assai es ning along opposite sides of the inner furnacewalls. Provision is made for the introduction of a carbonaceousatmosphere into the furnace through an inlet pipe 7 which extendsthrough the top of the furnace as shown in Fig. 1. The pressure of thefurnace atmosphere is preferably maintained slightly above theatmospheric pressure outside the furnace so that any leakage will takeplace out of the furnace rather than into the furnace, which latteroccurrence would result in contamination of the furnace atmosphere. Aleakage vent 8 may be provided to allow initial purging of the furnaceinterior and to permit a slow leakage during operation thereby providingfor a gradual and continuing replacement of spent gases. A thermocouple9 may also be employed to sense the temperature of the furnaceatmosphere and to actuate a suitable control arrangement for controllingthe heating current to the heaters o.

VThe heaters 6 are connected to an electrical power source by means ofsuitable leads 10 connected to terminal members l1 which extend throughstufling boxes f2 and into the interior of the furnace as shown. rTheterminal members il are electrically connected to opposite ends of theribbon resistors 6 as shown in Fig. 2.

Furnaces of the general type thus far described are well known in theart and are employed to perform a wide variety of heating and heattreatment operations. As has been pointed out, in applications wherecarbonaceous atmospheres are employed, heavy carbon deposits tend toform on the metallic conductors and terminals within the furnace wallsand also on those elements and fixtures operating in temperature zoneswithin the range of approximately 800 F. to 1300 F.

1n carrying my invention into elfect in the particular embodiment setforth herein, I provide, as shown in Figs, 3 and 4, a non-catalyticouter surface formed of a plating 13 of chromium on those portions ofthe terminal members 11 which extend through the furnace walls into theinterior of the furnace enclosure. As stated above, I have found thatchromium is non-catalytic to the carbon drop out reaction and that byproviding a chromium plating on the metallic lixtures in the furnace,such as the terminals lll, the problem of formation of free carbondeposits on these fixtures is, for all practical purposes, eliminated.

I have found that any thickness of plating may be employed with goodresults but I prefer to utilize a plating thickness in excess of .001since coatings of lesser thicknesses are subject to damage in normalhandling. l have obtained very satisfactory results with a chromiumcoating of about .005" applied in any well known manner on a standardhigh temperature conductor or terminal material such as cold rolledsteel, in which case the rate at which the carbon drop out reactionoccurred was reduced to a negligible value.

My invention may also be employed in connection with eliminating theproblem of carbon deposits on heating elements, either of the exposedtype or of the sheathed type, and in connection with various otherfixtures forming a part of the permanent structural framework of thefurnace interior.

It will be observed that one of the advantages of the particularembodiment of my invention illustrated and described herein resides inthe fact that with such an arrangement, the conducting core material maybe independently selected from the standpoint of providing the mostdesirable electrical characteristics taking into account the applicablecost considerations, without encountering the heretofore existingproblem of excessive carbon deposits.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

1. An electrically heated furnace comprising a heating enclosure forreceiving articles to be treated, means for introducing into saidenclosure a carbonaceous atmosphere containing carbon monoxide, heatingmeans corn- .i prising electrical heating elements mounted within saidenclosure, and conductor elements extending into said enclosure andconnected to said heating elements, said conductor elements having anexterior surface exposed to said atmosphere formed of chromium wherebycarbon drop out on said conductor elements from said carbonaceousatmosphere is substantially eliminated.

2. ln combination with an electric furnace having means 'introducingtherein a carbonaceous atmosphere conig carbon monoxide, an electricalconductor ele* ment outer surface exposed to said atmosphere in saidfurnace, at least a portion of said outer surface being formed ofchromium whereby carbon drop out on said conductor element from saidcarbonaceous atmosphere is substantially eliminated.

3. An electrically heated furnace comprising a heating enclosure forreceiving articles to be treated, means for introducing into saidenclosure a carbonaceous atmosphere containing carbon monoxide, heatingmeans comprising electrical heating elements mounted within saidenclosure, terminal means exterior of said furnace connectible to apower source for energizing said heating elements, and conductor meansconnected to said terminal means and extending into said enclosure fromthe exterior of said furnace and through a zone wherein the temperatureof operations is to be Within a range wherein carbon deposits are likelyto form on said conductor means at an objectionable rate, said conductormeans having an outer surface formed of chromium at least within saidzone whereby forming of said carbon deposits on said conductor means issubstantially eliminated.

4. ln combination with an electric furnace having means for introducingtherein a carbonaceous atmosphere containing carbon monoxide, acomposite electrical conductor element comprising an inner core ofelectrical conducting material selected from the standpoint of providingthe proper electrical characteristics, and an outer plating on said coreof chromium for reducing the rate of carbon formation on said conductorelement.

5. An electrically heated furnace comprising a heating enclosure forreceiving articles to be treated, means for introducing into saidenclosure a carbonaceous atmosphere containing carbon monoxide, heatingmeans comprising electrical heating elements mounted within saidenclosure, conductor elements extending into said enclosure andconnected to said heating elements, and a coating of chromium over atleast a portion of said conductor elements for reducing the rate ofcarbon deposit thereon.

6. An electrically heated furnace comprising a heating enclosure forreceiving articles to be treated, means for introducing into saidenclosure a carbonaceous atmosphere containing carbon monoxide, heatingmeans cornprising electrical heating elements mounted within saidenclosure, electrical terminal means exterior of said furnaceconnectible to a power source for energizing said heating elements,conductor means connected to -said terminal means and extending intosaid enclosure from the exterior of said furnace and through a zonewithin said furnace wherein the temperature of operation is to be Withina range wherein carbon deposits are likely to form on said conductormeans at an objectionable rate, and an outer coating of chromium on saidconductor means within said zone for reducing the rate of carbonformation on said conductor means.

References (Jited in the le of this patent UNITED STATES PATENTS1,360,267 Chubb Nov. 30, 1920 1,538,972 Conradty May 26, 1925 1,624,345Keene Apr. 12, 1927 1,764,045 Kelleher June 17, 1930 2,009,732 Harper etal. July 30, 1935 2,232,327 Hubald Feb. 18, 1941 2,359,157 Roth Sept.26, 1944

5. AN ELECTRICALLY HEATED FURNACE COMPRISING A HEATING ENCLOSURE FORRECEIVING ARTICLES TO BE TREATED, MEANS FOR INTRODUCING INTO SAIDENCLOSURE A CARBONACEOUS ATMOSPHERE CONTAINING CARBON MONOXIDE, HEATINGMEANS COMPRISING ELECTRICAL HEATING ELEMENTS MOUNTED WITHIN SAIDENCLOSURE, CONDUCTOR ELEMENTS EXTENDING INTO SAID ENCLOSURE ANDCONNECTED TO SAID HEATING ELEMENTS, AND A COATING OF CHROMIUM OVER ATLEAST A PORTION OF SAID CONDUCTOR ELEMENTS FOR REDUCING THE RATE OFCARBON DEPOSIT THEREON.